A cell wall fraction of the yeast phase cells of Blastomyces dermatitidis has been isolated and utilized in an evaluation of delayed type hypersensitivity (DHS) of blastomycosis. Chemical analysis of the DHS reactive component has been studied. Three parameters are being further investigated in a effort to compare the efficacy of the new antigen to various filtrates of mycelial and yeast phases of B. dermatitidis: skin test sensitivity, migration inhibition assay (MIF) and lymphoblast transformation studies using the incorporation of radioactive precursors. The antigen has shown to be effective in skin test reactivity in Blastomyces-infected guinea pigs. In vitro analyses, including migration inhibition assay of peritoneal macrophages and lymphocyte transformations of lymph node homogenates, will be studied. The preliminary studies suggest a specific and sensitive antigenic component that may be useful in the diagnosis and epidemiological investigation of blastomycosis.